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Gasoline: How does the filling gun know that the gas tank is full?

Gasoline is a toxic, clear liquid. We mostly use it as a fuel in internal combustion engines. Also people can buy it at gas stations. In addition, the most common fuels are gasoline and diesel fuel. The first filling station was in Germany. Where Bertha Benz refilled the tank of the first automobile in 1888.


Gasoline gun

Gasoline vapors are explosive. And their concentration in the filler cap of the car is maximum. So we can not use an electric sensor in this case. Therefore, the fuse of the filling gun works on pure mechanics.


By pressing the lever, we push up the valve stem (blue). In that way we open the way to the gasoline tank. Note that the axis of the lever is attached to the stem of the fuse (green).


Gasoline flows through a narrow ring. In the area of narrowing the flow rate increases. And the pressure in the liquid drops.  This is a consequence of the Venture effect. And we call the device itself the Venture ring. Also it has holes communicating with the air chamber (green). Fluid flow creates a vacuum in the air chamber. People also used the Venture effect in spray guns, airbrushes, and carburetors.


The spout of a filling pistol has a small hole. It is also connected to the air chamber. The venture ring draws air out of the chamber. But the atmospheric air through the sensor hole immediately fills its supply. So there is no vacuum in the chamber.

As soon as the fuel level in the tank rises above the sensor hole, the flow of atmospheric air into the chamber stops. The resulting vacuum pulls in the membrane, which pushes the fuse rod down.

With a characteristic click, the base of the lever moves away from the stem of the check valve. And the flow of fuel is blocked.

In addition, gasoline hurts the environment and human health. It is harmful if a human drinks it. Or if it gets on the skin.

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